Currently it appears the backpack can have it's ability enabled indefinitely without costing anything. This seems pretty overpowered and unbalanced.

I'd recommend implementing a cost for the ability to be enabled, whether that's draining your EXP, consuming items, consuming a fluid from the tank, etc.

This way players would have to at least put in some effort in order to keep benefitting from the backpack ability.

The experience consumption seems like a good idea, I might start working on it. It'll be configurable

#889

Yes please add this. And also the more OP ones can simply cost/consume more making it more balanced overall. Here are a few ideas:
-Squid backpack gradually fills with water when underwater and only gives the boost as long as the backpack is not full (Sure you could use a bucket to empty it but if you dont have one then yea)
-Same for magma cube pack (fills with lava and only gives resistance as long as its not full).
-Spider backpack consumes a string every time you start climbing (consumes one every second to maintain climbing?)
-Ore backpacks (+armor) are fine as is and dont need to consume anything
-etc, may add more as I think of some

But can't you empty the tanks with shift-clicking them?

(Maybe that feature would be removed?)

Just wanted to second this. As of right now, the squid and magma cube backpacks in particular are quite overpowered.